Strut suspension systems for automobiles are used to suspend steered wheels and include, for example, one having such a structure as illustrated in FIG. 3. In this strut suspension system, a knuckle 2 connected to a wheel 1 via a bearing is connected to a vehicle body 7 by a lower arm 3 and a strut 4.
As is illustrated in FIG. 3, the strut 4 is provided with a shock absorber 5 having a cylinder tube 5a and a piston rod 5b, and a coil spring 6 is arranged around the shock absorber 5. The cylinder tube 5a is connected at a lower end portion thereof to an upper part 2B of the knuckle 2, and an upper end portion of the piston rod 5b, said upper end portion extending upwardly from an upper end portion of the cylinder tube 5a, is connected at a top end thereof to the vehicle body 7. The coil spring 6 is connected at a lower end thereof to an outer circumference of the cylinder tube 5a, and is connected at an upper end thereof to the vehicle body 7.
The strut suspension system illustrated in FIG. 3 is of the single-path top mount structure that the piston rod 5b and the coil spring 6 are integrally connected with each other at the upper ends thereof. From an upper part of the strut 4 to the vehicle body 7, are action force from a road surface is inputted along a single path after its transmission by way of the shock absorber 5 and the coil spring 6.
Described specifically, an upper spring seat 8a is fixedly arranged on an outer circumference of the upper end of the piston rod 5b, and the upper end of the coil spring 6 is connected to the upper spring seat 8a and is hence connected integrally with the upper end of the piston rod 5b. The piston rod 5b is connected at the upper end thereof to the vehicle body 7 via a bearing 9a and an insulator 10a. Incidentally, the coil spring 6 is connected at the lower end thereof to the lower spring seat 11 fixedly arranged on the outer circumference of the cylinder tube 5a. 
Illustrated in FIG. 4, on the other hand, is a strut suspension system of the dual-path top mount structure that an upper end of a piston rod 5b and an upper end of a coil spring 6 are independent from each other. From an upper part of a strut 4 to a vehicle body 7, a reaction force from a road surface is inputted along dual paths via a shock absorber S and a coil spring 6.
Described specifically, a plate 12a is fixedly arranged on an outer circumference of the upper end of the piston rod 5b, an insulator 10b is disposed on a strut attachment portion of the vehicle body 7 such that the plate 12a is held on upper and lower sides thereof by the insulator 10b, and the piston rod 5b is connected at the upper end thereof to the vehicle body via the insulator 10b. Below the insulator 10b on the strut attachment portion of the vehicle body 7, on the other hand, a spring seat 8b is arranged with a bearing 9b interposed between the spring seat 8b and the insulator lob, and the coil spring 6 is connected at the upper end thereof to the spring seat 8b. Incidentally, the coil spring 6 is at an lower end thereof to a lower spring seat 11 fixedly arranged on an outer circumference of a cylinder tube 5a, as in the strut suspension system of the single-path top mount structure (see FIG. 3).
The wheel 1 is steered about a king pin axis 20 as illustrated in FIG. 3. The king pin axis 20 is a straight line, which extends between an upper support point Pa of the strut 4 and a lower support point Pb of the knuckle 2. The strut 4 and the knuckle 2 are pivotally supported at the upper support point Pa by the insulator 10a (10b) and the bearing 9a (9b) and at the lower support point Pb by an unillustrated ball joint, respectively, such that the strut 4 and the knuckle 2 are both rotatable about the king pin axis 20.
For the structural constraints around the wheel, it is difficult to have the king pin axis 20 and an axis 21 of the strut 4 coincided with each other. No matter whether a strut suspension system is of the single-path top mount structure or of the dual-path top mount structure, the king pin axis 20 is generally inclined toward the outboard side of the vehicle 7 relative to the axis 21 of the strut 4 as depicted in FIG. 3.
Further, a plane 13a of rotation of the spring seat 8a (8b), said plane 13a being equivalent to a plane of rotation of the bearing 9a (9b) in the illustrated prior art strut suspension systems, is generally set to lie in a plane which is perpendicular to the axis 21 of the strut but is inclined (not perpendicular) relative to the king pin axis 20.
In the strut suspension system of the dual-path top mount structure that the top end of the piston rod 5b and the top end of the coil spring 6 are independent from each other, a moment (steer moment) which causes the strut 4 to rotate is therefore produced by spring reaction force from the coil spring 6 although such a moment does not occur in the strut suspension system of the single-path top mount structure that the piston rod 5b and the coil spring 6 are integrally connected together at the upper ends thereof. This moment acts as a cause of a deflection of a vehicle and has posed a problem.
Factors of occurrence of such a steer moment will now be discussed. A discussion will firstly be made about a left wheel with reference to FIG. 5A and FIG. 5B. Reaction force from the coil spring 6 to the side of the vehicle body 7 is applied toward an outer side of a line, which extends between an application point (upper point of application of force) P1 and an application point (lower point of application of force) P2, at both of the force application points.
Now imaging an x-y-z coordinate system, in which the king pin axis 20 extends as a z-axis and, in a plane lying at a right angle relative to the z-axis (king pin axis 20) and including the upper force application point P1, the longitudinal direction of the vehicle body extends as an x-axis and the lateral direction of the vehicle body extends as a y-axis, the x,y-coordinates of the upper force application point P1 is (x,y) Further, x,y components of applied force F at the upper force application point P1 will be designated as (Fx,Fy).
Also imagine an x′-y′-z′ coordinate system, in which the axis 21 of the strut 4 extends as a z′-axis and, in a plane lying at a right angle relative to the z′-axis and including the upper force application point P1 (which plane serves as a plane of rotation for the upper spring seat 8), the longitudinal direction of the vehicle body extends as an x′-axis and the lateral direction of the vehicle body extends as a y′-axis. Assuming that an angle between the z-axis (king pin axis 20) and the z′-axis [the axis 21 of the strut 4 which lies at a right angle relative to the plane 13 of rotation of the upper spring seat 8b] is θ and also that an offset of the z′-axis (axis of rotation of the upper spring seat 8b) in the direction of the y-axis relative to the z-axis (king pin axis 20) on the x-y plane is δ [see FIG. 5C; δ generally takes a negative value], the x′y′-coordinates of the upper force application point P1 is expressed by:(x,(y−δ)·cos θ)≈(x,y−δ) (∵θ: very small)
Expressing the components (Fx′,Fy′) in the x′-y′ plane (the plane of rotation of the upper spring seat 8b) of the applied force F at the upper application point P1 by using x and y, these components can be defined as follow:(Fx,Fy·cos θ+Fz·sin θ) [see FIG. 6B]
Accordingly, a moment M1 about the axis of rotation of the upper spring seat 8b (z′-axis) by the applied force F at the upper force application point P1 can be expressed as follow:M1=x·(Fy·cos θ+Fz·sin θ)−(y−δ)·Fx  (1)
This moment M1 is transmitted downwardly via the coil spring 6. If the coil spring 6 is taken as a universal joint, the moment M1 acts approximately as a moment about the king pin axis 20.
Supposing that the distance between the upper force application point P1 and the lower force application point P2 is H, the x,y-coordinates of the lower force application point P2 in the x-y-z coordinate system are:(x−Fx/Fz·H,y−Fx/Fy·H)Components (x,y components) of applied force F at the lower force application force P2, said components being perpendicular to the z-axis (king pin axis 20), are:(−Fx,−Fy)
Therefore, a moment M2 about the axis of rotation of the lower spring seat 11 (z-axis) by the applied force F at the lower force application point P2 can be expressed as follow:                                                         M2              =                            ⁢                                                                    -                                          (                                              x                        -                                                                              Fx                            /                            Fz                                                    ·                          H                                                                    )                                                        ·                  Fy                                +                                                      (                                          y                      -                                                                        Fx                          /                          Fy                                                ·                        H                                                              )                                    ·                  Fx                                                                                                        =                            ⁢                                                                    -                    x                                    ·                  Fy                                +                                  y                  ·                  Fx                                                                                        (        2        )            
As a steer moment ML by a reaction force from the coil spring 6 via the upper and lower spring seats is the sum of M1 and M2,                                                                         M                L                            =                                                x                  ·                                      (                                                                                            Fy                          ·                          cos                                                ⁢                                                                                                   ⁢                        θ                                            +                                                                        Fz                          ·                          sin                                                ⁢                                                                                                   ⁢                        θ                                                              )                                                  -                                                      (                                          y                      -                      δ                                        )                                    ·                  Fx                                -                                  x                  ·                  Fy                                +                                  y                  ·                  Fx                                                                                                        =                                                x                  ·                                      (                                                                  Fy                        ·                                                  (                                                                                    cos                              ⁢                                                                                                                           ⁢                              θ                                                        -                            1                                                    )                                                                    +                                                                        Fz                          ·                          sin                                                ⁢                                                                                                   ⁢                        θ                                                              )                                                  +                                  δ                  ·                  Fx                                                                                        (        3        )            
The steer moment ML by the reaction force from the coil spring 6 as described above is applied to the left wheel. When the same strut assembly is commonly used for both the left and right wheels (in other words, the coil spring 6 is used commonly on both sides), a steer moment MR applied to the right wheel is the same as ML applied to the left wheel (see FIG. 7).MR=x·(Fy·(cos θ−1)+Fz·sin θ)+δ·Fx  (4)
Therefore, a total steer moment M by the reaction force from the coil spring 6 is:                                                         M              =                                                M                  L                                +                                  M                  R                                                                                                        =                              2                ·                                  (                                                            x                      ·                                              (                                                                              Fy                            ·                                                          (                                                                                                cos                                  ⁢                                                                                                                                           ⁢                                  θ                                                                -                                1                                                            )                                                                                +                                                                                    Fz                              ·                              sin                                                        ⁢                                                                                                                   ⁢                            θ                                                                          )                                                              +                                          δ                      ·                      Fx                                                        )                                                                                        (        5        )            
Steer moments applied to the suspension systems of a vehicle include, in addition to reaction forces from the coil springs 6 of the suspension systems for both the left and right wheels, those produced by tire reaction forces MTL,MTR from the left and right wheels as illustrated in FIG. 7. As these tire reaction forces MTL,MTR are cancelled out by the left and right wheels, it is a steer moment produced by reaction force from the coil spring 6 that causes a deflection of the vehicle.
Deflection of the vehicle can, therefore, be suppressed if the total steer moment M by reaction force from the coil spring 6 can be reduced.
The total steer moment M depends significantly upon the x-coordinate x of the upper force application point P1 (namely, the longitudinal offset of the upper force application point P1 relative to the king pin axis 20) and the angle θ between the z-axis (king pin axis 20) and the z′-axis (the axis 21 of the strut 4) (in other words, the angle formed between the plane, which lies at a right angle relative to the king pin axis 20, and the plane 13a of rotation of the upper spring seat 8b).
If the longitudinal offset x of the upper force application point P1 relative to the king pin axis 20 is reduced, the total steer moment M is rendered smaller, thereby making it possible to suppress deflection of the vehicle. This, however, requires extremely difficult work that the positioning of the upper force application point 1 is performed in the manufacturing process. Therefore it is not easy to reduce the offset x.
JP 2,715,666 B discloses a strut suspension system in which an axis of rotation of a rolling bearing on an upper part of a strut is arranged coaxially with a king pin axis. Since this strut suspension system is of the single-path top mount structure, spring reaction force does not become a cause of occurrence of a steer moment, and the problem that is to be solved by the present invention does not arise. The invention disclosed in this Japanese patent publication is, therefore, different in technical field different from the present invention.
U.S. Pat. No. 5,454,585, on the other hand, discloses a strut suspension system with dual-path top mounts constructed such that, as shown in FIG. 8 of this application, a plane of rotation of an upper spring seat 110 is arranged to lie substantially at a right angle relative to a king pin axis 130 and that an axis of rotation of a rolling bearing (bearing assembly) 120 at an upper part of a strut is substantially coaxial with the king pin axis 130. According to this construction, deflection of a vehicle can be suppressed by reducing a total steer moment which applies to the suspension system.
The bearing assembly 120 is composed of an upper, stationary-side member 122, a lower, rotating-side member 124, and balls 126 interposed between these upper and lower members. A suspension spring 140 is connected at an upper end thereof to an upper spring seat 110 via a rubber seat 142. Further, a vibration isolating rubber 152 is arranged on an upper end portion of a piston rod 150 of a shock absorber, and the vibration isolating rubber 152 is covered around a circumference thereof by a dust cover 154. The piston rod 150 is connected at an upper end portion thereof to a vehicle body via a rubber body 180 with core members 182,184 embedded therein.
According to this technique, however, a spacer (wedge) 170 of a wedge shape which corresponds to an inclination of an axis 156 of the shock absorber and that of the king pin axis 130 is interposed between a mount surface 160 of the rubber body 180 and the bearing assembly 120 to adjust the angle of the upper spring seat 110 and that of the bearing assembly 120. In other words, the upper spring seat 110 and the bearing assembly 120 are connected to the vehicle body via the wedge 170 and the mount surface 160 of the rubber body 180. Therefore the wedge 170 is newly required, leading to an increase in the number of parts.
Further, the bearing assembly 120 is arranged on the upper spring seat 110 such that they are substantially in series with each other. This arrangement is, however, disadvantageous in decreasing the axial length of the suspension and increasing the effective length of the suspension.
The present invention has been completed with the foregoing problems in view, and as objects, has the provision of a structure that in a strut suspension system with dual-path top mounts, a total moment applied to the suspension system can be reduced to suppress deflection of a vehicle without an increase in the number of parts and also the provision of a structure that in a strut suspension system with dual-path top mounts, the axial direction and effective length of the suspension system can be advantageously reduced and increased, respectively.